The present invention relates to measurement devices and circuits and, in particular, a circuit for reducing the 1/f noise level of the detection circuit.
In most non-dispersive infrared detection applications (NDIR) using thermal detectors, the radiation source is modulated either because of the AC nature of the detector or to reduce the effects of stray radiation or temperature drifts. The infrared source could also be modulated by some mechanical means, such as a chopper wheel connected to a constant RPM motor to alternately pass and block the infrared radiation. Such an approach used with pyro-electric detectors requires the use of somewhat expensive precision motors, controllers, and chopper wheels precisely synchronized to provide the modulation and demodulation of signals.
In order to overcome the difficulties with such detection systems, it is desirable to use a DC system in which a thermopile is provided to detect the infrared radiation passing through an analyte, however, the use of such a detector requires a DC amplifier for providing a signal level which is representative of the nature of the analyte being detected as well as its concentration. DC amplifiers can be employed for such purpose, however, DC amplifiers inherently have internal noise referred to as 1/f noise which exponentially decreases with increasing frequency. Thus, at very low frequencies, the 1/f noise is significant and for use in NDIR systems with DC emitters, such noise adversely affects the resultant detected signal. The 1/f noise, however, decreases significantly at higher frequencies, such as 1 kHz and is negligible at or above such frequency.